A great number of complicated hydrocarbons play major roles in biological recognition processes such as recognitions between cells, cell growth and cell differentiation (non-patent reference 1). They compose blood type determinants (non-patent reference 2) and form cancer-related antigens (non-patent reference 3). In the plant kingdom, they exert coordination functions as hormones (non-patent reference 4) and form binding sites to lectin (non-patent reference 5).
For example, modified sugars such as deoxy- and fluoro-sugars and epimers of naturally occurring sugars provide important approaches for research works about interactions of them. Absolutely in the same manner as in the case of the interactions between specific enzymes and substrates, general rules about the interactions between proteins and carbohydrates are mostly focused in their association, interestingly. In other words, the information about the activity center of an enzyme can be obtained by continuous modifications of a substrate for the enzyme. Furthermore, attention is focused on deoxyglycosides, in particular, because of the fact that deoxyglycosides exist in a great number of antibiotics (non-patent reference 6).
It is reported that 2-deoxyglucose inhibits glycolysis in cancer cells and proliferation of cancer cells. It is also reported that 2-deoxyglucose delays cancer proliferation in some animal models. Additionally, research works are under way about combinations of other cytokines with anti-cancer drugs (patent reference 1). As described above, it is expected that deoxyhexose may be applicable to research works about metabolism and biological signals, in particular.
Izumori Ken as one of the inventors publicly discloses the Izumoring coordination scheme about tetrose, pentose and hexose in the patent reference 2 and describes the usefulness thereof. Specifically, the Izumoring coordination scheme is a scheme totally showing the coordination among all monosaccharides with 4 to 6 carbon atoms by linking them via production processes and molecular structures (D forms, L forms) as shown in FIG. 4. In other words, what is indicated in FIG. 4 is that all monosaccharides with 4, 5 and 6 carbon atoms are linked together. The whole scheme shows the linking in the Izumoring C6, the linking in the Izumoring C5, the linking in the Izumoring C4, and the linking of C4, C5 and C6 together. The concept is very important. So as to reduce the number of the carbon atoms, mainly, fermentation processes are used. The Izumoring scheme is characteristically a significant coordination scheme linking all monosaccharides with different carbon atoms together.
As shown in the lower column in FIG. 4, and in FIG. 5 and FIG. 8, the Izumoring of monosaccharides with 6 carbon atoms (hexose) shows that monosaccharides with 6 carbon atoms (hexose) include 34 types in total, where aldose includes 16 types; ketose includes eight types; and sugar alcohol includes 10 types. Rare sugars are defined as monosaccharides (aldose and ketose) and derivatives thereof (sugar alcohol) which exist rarely in the natural kingdom. Since the definition is not a definition according to the sugar structure or the sugar properties, the definition itself is so ambiguous. In other words, no definition about the amount such that rare sugars exist at a certain level or less is given. However, the aldose abundantly existing in the natural kingdom includes six types of D-glucose, D-galactose, D-mannose, D-ribose, D-xylose and L-arabinose. Therefore, aldose types except those described above are defined as rare sugars. D-Fructose exists as the ketose, while ketose types except D-fructose are defined as rare sugars. The other ketose types include D-psicose, D-tagatose, D-sorbose, L-fructose, L-psicose, L-tagatose and L-sorbose. Meanwhile, sugar alcohol is prepared by reducing monosaccharides. In the natural kingdom, D-sorbitol exists relatively abundantly, but sugar alcohols except D-sorbitol exist at so smaller amounts quantitatively that these sugar alcohols are also defined as rare sugars.
It has been known by research works including research works of the inventors that these sugars can be converted via redox enzyme reactions, aldose isomerase reactions, and aldose reduction enzyme reactions. D-Glucose (grape sugar) and D-fructose exist abundantly in the natural kingdom and are inexpensive, but these rare sugars have never been synthetically prepared yet. Herein, a novel enzyme was discovered. Quite unexpectedly, D-sorbose was found in a liquid culture of a bacterium generating an enzyme synthesizing D-sorbose from galactitol. That was the beginning. The cause was examined. Consequently, it was found that the bacterium generated an enzyme D-tagatose 3-epimerase (DTE) (patent reference 3). It is indicated that DTE is an enzyme linking between D-tagatose and D-sorbose, which have never been linked together. More surprisingly, DTE is an enzyme epimerizing all ketose types at position 3, and having such wide substrate specificity to interact with D-fructose and D-psicose, L-sorbose and L-tagatose, D-tagatose and D-sorbose, and L-psicose and L-fructose, which have never been synthetically connected together. In other words, it is indicated that the enzyme is a unique enzyme capable of selecting a substrate in a very wide range. Via the discovery of DTE, all monosaccharides are linked together in a ring shape, so that information about monosaccharides can be structurally constructed, which was designated Izumoring.
In a thorough view of FIG. 5, L forms are shown on the left side; D forms, on the right side; and DL forms, on the center. Additionally, L forms and corresponding D forms thereto locate to each other in point symmetry on the center (asterisk) of the ring. For example, D-glucose and L-glucose position to each other in point symmetry on the center point as the base. The value of Izumoring additionally resides in the design scheme for producing all monosaccharides. When intending to produce L-glucose from a starting material D-glucose, in the previous example, Izumoring shows that L-glucose can be prepared from D-glucose through isomerization, epimerization, reduction, oxidation, epimerization and isomerization.
Using Izumoring about monosaccharides with 6 carbon atoms (hexose), relations between sugars abundantly existing in the natural kingdom and rare sugars existing at a trace of amounts are shown. D-Glucose, D-fructose, D-mannose and D-galactose generated from lactose in cow milk exist abundantly in the natural kingdom, while sugars except those described above are grouped as rare sugars existing at extremely small amounts. The discovery of DTE enabled the production of D-fructose and D-psicose from D-glucose and also the production of D-allose, allitol and D-talitol. A rare sugar D-psicose has hardly been available so far, but a method for producing rare sugars from monosaccharides abundantly existing in the natural kingdom is now being developed. By utilizing the technique, the rare sugar D-psicose can be produced.
Herein, the meaning of Izumoring about monosaccharides with 6 carbon atoms is now described collectively below: all monosaccharides are structurally coordinated (structurally constructing information) via production processes and molecular structures (D forms, L forms) to catch the whole image of monosaccharides; an effective and efficient approach about research works thereof can be selected; an optimal production route can be designed; and defective parts may be anticipated.
Izumoring about monosaccharides with 5 carbon atoms (pentose) is of a ring smaller than the ring of Izumoring about monosaccharides with 6 carbon atoms, as shown in the middle column of FIG. 4 and in FIG. 6. Nonetheless, the Izumoring includes all of eight aldose types, four ketose types and four sugar alcohols, like C6 Izumoring, so that all monosaccharides are linked together via enzyme reactions. All the monosaccharides are linked together in a ring shape in the same manner except for a different point that the monosaccharides are produced only by redox reaction and isomerization reaction. It is indicated that by using DTE, a more efficient production route can be designed. As apparently shown in FIG. 6, in particular, the characteristic feature of the Izumoring with 5 carbon atoms is the symmetry on the right and left sides in contrast to the Izumoring with 6 carbon atoms, where all monosaccharides are arranged in point symmetry. Since all these pentose types are linked together with enzyme reactions, all the pentose types are structurally arranged (information construction) in the same manner as in the case of the Izumoring with 6 carbon atoms, so that the whole image can be given; an effective and efficient approach for research works about such pentose types can be selected; the optimal production route can be designed; and a defective part can be anticipated, significantly.
As shown in the upper column of FIG. 4 and FIG. 7, the Izumoring about monosaccharides with 4 carbon atoms (tetrose) cannot form a complete ring due to the characteristic structural feature of tetrose. The Izumoring with 4 carbon atoms has a structure corresponding to the upper half of the Izumoring with 5 carbon atoms. The ring is also in linking via redox and isomerization reactions absolutely in the same manner as the Izumoring with 5 to 6 carbon atoms does. Because DTE never reacts with ketose with 4 carbon atoms, currently, no reaction exists between ketose types. However, the existence of a novel epimerase is anticipated, so research works about the existence thereof are now under way.
The whole arrangement is in symmetry on the right and left sides, like the arrangement in the Izumoring with 5 carbon atoms and includes all four aldose types, two ketose types and three sugar alcohol types. In other words, the same meaning as the Izumoring with 5 and 6 carbon atoms exists.
D-Glucose in the Izumoring C6 is linked to D-arabitol in the Izumoring C5 and to erythritol in the Izumoring C4. The lines show that D-arabitol and erythritol can be produced from D-glucose by a fermentation method. In other words, the Izumoring C6, the Izumoring C5 and the Izumoring C4 are linked together. The linking is mainly due to reactions by fermentation, causing the reduction of carbon atoms. The Izumoring C6, the Izumoring C5 and the Izumoring C4 may be linked together by fermentation methods other than the two conversion reactions to D-arabitol and erythritol. For example, D-ribose may be produced from D-glucose.
As described above, all monosaccharides (aldose, ketose and sugar alcohol) with 4, 5 and 6 carbon atoms are linked together via the three Izumoring schemes. Therefore, the locations of individual monosaccharides in all the monosaccharides can be clearly identified. It is readily confirmed that the most famous Xylitol can be produced readily by reducing D-xylose prepared from wood pulp as a source not utilized.
In case that a specific monosaccharide is obtained abundantly via a reaction in a biological organism, the conversion of the specific monosaccharide to a novel monosaccharide will possibly be found readily. Because the locations of all monosaccharides as raw materials can securely be obtained on the basis of the whole image, a useful method for the utilization thereof can be designed. Particularly, the method for using any monosaccharide obtained from wastes or by-products can readily be anticipated.
When monosaccharides are reduced, aldehyde group and ketone group therein are converted to alcohol group, so that polyhydric alcohols with the same carbon atoms, namely sugar alcohols are produced. Many of reducing sugars are useful in fields of food, etc.; for example, L-arabinose as a pentose has a taste close to sucrose and is slightly absorbed, so L-arabinose is a non-caloric sugar. Additionally, it is known that disaccharides such as sucrose and maltose inhibit disaccharide hydrolases in exerting its function during the absorption thereof into biological bodies. It is expected that such disaccharides may be utilized as diet sweeteners or sweeteners for diabetic patients. Additionally, L-arabinose is a useful sugar as a raw material for synthetically preparing pharmaceutical products.
In case of intending to obtain reducing sugars, the origin may be screened for in naturally occurring materials. For example, recently, a method for producing L-arabinose comprising allowing an enzyme and an acid to react with corn fiber, gum Arabic and beet pulp has been developed as an approach for obtaining L-arabinose. Crude fibers such as arabinan, arabinoxylan and arabinogalactan as raw materials frequently exist in mixtures with for example pectin and unnecessary crude fiber. In solutions resulting from the enzyme decomposition and acid hydrolysis of them, pectin, crude fibers or decomposition products thereof in addition to reducing sugars such as L-arabinose are mixed together and concurrently exist. As to a method for purifying L-arabinose, there are proposed for example a method of fractionation of the intended L-arabinose from xylose and oligosaccharides in an L-arabinose-containing sugar solution by chromatography with ion exchange resins; a method by chromatography with ion exchange resins for the purpose of separating L-arabinose from polysaccharides, oligosaccharides and salts; and film treatment.
As to deoxy monosaccharides, alternatively, effective methods for producing such deoxy monosaccharides and substances recognized as deoxy monosaccharides are very few. Therefore, the establishment of a method for producing deoxy monosaccharides is first desired.    Patent reference 1: JP-T-2006-515883 (the term “JP-T” means a published Japanese translation of a PCT patent application)    Patent reference 2: WO 2004/063369    Patent reference 3: Japanese Patent 3,333,969    Non-patent reference 1: G. E. Edelman, Spektrum Wiss. 1964 (6), 62    Non-patent reference 2: V. Ginsburg, Adv. Enzymol. 36, (1972), 131    Non-patent reference 3: G. M. W. Cook, E. W. Stoddard, “Surface carbohydrates of the Eucaryotic Cell”, Academic Press, London, 1973    Non-patent reference 4: P. Albersheim, A. G. Darvill, Spektrum Wiss. 1985(11), 86    Non-patent reference 5: T. W. Rademacher, R. B. Parekh, R. A. Dwek, Ann. Rev. Biochem., 57, (1988), 785    Non-patent reference 6: T. Reichstein, E. Weiss, Adv. Carbohydr. Chem. 17, (9162[sic]), 65